The invention relates to a reproducing apparatus for demodulating a head read signal by using a partial response maximum likelihood detection and, more particularly, to a reproducing apparatus of a magnetic disk apparatus or the like having a demodulating circuit of a partial response class 4 and a demodulating circuit of an extended partial response class 4.
In recent years, a high speed and a miniaturization of a computer have been advanced. Even in a magnetic disk apparatus which is a main stream in external memory apparatuses of computers, similarly, a request for realizing a high speed and a miniaturization of the apparatus is increasing. In response to such a request, in order to realize the high speed and the miniaturization of the magnetic disk apparatus, various kinds of developments of an LSI to realize a high speed transfer and developments and improvements of a head and a magnetic disk medium to accomplish a high recording density have been performed. As a method of accomplishing such high speed transfer and high density recording of the magnetic disk apparatus, in recent years, a partial response system has been developed and the use of such a system in a demodulating circuit of read data has been started. However, it is further necessary to improve the high speed transfer and high density recording.
Hitherto, a peak detecting circuit has been used as a demodulating circuit of the magnetic disk apparatus. Recently, however, since a high speed of a circuit and a high density of a magnetic disk medium have been advanced, the demodulating circuit for the peak detection reaches a limitation, so that the use of a partial response class 4 maximum likelihood detection (hereinbelow, referred to as a "PR4ML") has been started in recent years. Even in the PR4ML demodulating circuit, when the realization of the high density of the magnetic disk medium is further progressed, there is a limitation even in case of further reducing a desired S/N ratio which is necessary to obtain a predetermined bit error rate by the maximum likelihood decoding. As a method of further improving the performance of such a PR4ML demodulating circuit, an extended partial response class 4 maximum likelihood detection (hereinbelow, referred to as an "EPR4ML") has been proposed.